The present invention relates to precision measurement of satellite location and, more particularly, to a method for adapting to or estimating the effects of satellite motion.
Conventional systems used to measure the precise location of a satellite employ a receiving antenna having a narrow beam for receiving a signal from the satellite. The system measures the pointing error by some means, and then makes a correction. One method for measuring the pointing error is to use a monopulse tracking system or a pseudo monopulse system which has an antenna pattern having a sharp null. The pointing of the antenna is adjusted continually to try to keep the signal source in the null.
Another method for measuring the pointing error is to observe the amplitude modulation as a narrow beam antenna is conically scanned around the bearing to the satellite. When the signal amplitude is highest, the antenna is pointed at the satellite. When the amplitude decreases, the pointing error is increasing. In a step scan system, the angle between the antenna boresight and the line of sight vector may be determined by making four measurements around the best estimate of the line of sight. Given the best estimate of the line of sight to the satellite, the antenna is commanded to four points. The first step is up in elevation and to the right in azimuth, then up in elevation and to the left in azimuth, thirdly down in elevation and to the left in azimuth, and finally, down in elevation and to the right in azimuth. The received signal from the satellite is measured at each of these four positions. The four measurements taken at these four positions are combined to provide the measured azimuth and elevation error between the estimated line of sight and the true line of sight.
The current state of the art for steering antennas to point at a slowly moving satellite utilizes sequential comparison of amplitude between two points without employing any smoothing. The antenna is sequentially moved in the direction of increasing signal strength. This method is described in a technical paper entitled "An Improved Step-Track Algorithm for Tracking Geosynchronous Satellites" by M. Richharia in the International Journal of Satellite Communications, Vol. 4, pages 147-156 (1986).
Regardless of which type of measurement system is used, the distinguishing feature is that in conventional systems, the antenna is moved directly in response to a measurement. That is, the error is measured by some means or another, and then the error is corrected by moving the antenna by the amount necessary to correct the error. No smoothing is employed.
The problem is that the satellite is moving. Making the measurement, and moving the antenna thereafter in response to the measurement, takes a finite amount of time. By the time the process is completed, the satellite has moved. Thus, there is always an error. If the satellite were still, that is, not moving, after a number of measurements over a period of time, the pointing error can be made arbitrarily small. However, because the satellite is moving while the measurements are being made and the corrective movements of the antenna are being made, the error can only be resolved to about 1/10 of the beam width of the antenna. Sometimes the error can be made as small as 1/10 of a degree.
There is a need to have measurements as accurate as 1/100 of a degree to be used for making an ephemeris. Accordingly, it is an objective of the present invention to provide a precision satellite tracking system that can resolve satellite error to be in the order of hundredths of a degree.